1. Field of the Invention
This invention relates to a process for producing flat products from particulate material and to flat products produced by such a process. By the term "flat products" it is meant products in strip, sheet or like form or products produced therefrom which have retained a generally flat appearance.
2. Description of the Prior Art
A process for the production of strip from metal powder is known in which a suspension of powdered metal in a solution of a film-forming binder material in water is coated in the form of a slurry onto a support surface, dried and removed from the support surface as a thin, flexible strip. This strip is subsequently compacted within a rolling mill and sintered to produce the final strip product.
Hitherto, process operators have favoured the use of powders consisting, essentially, of irregular shaped particles as are produced, for example, by water atomisation techniques.
It has been established that these irregular shaped particles bind together more effectively than do spherical particles thereby producing relatively higher green strengths in the compacted strip. In addition the increased surface area of irregular particles provides greater particle contact area after compaction thereby increasing the surface area over which diffusion processes can occur during subsequent sintering resulting in greater strength for the sintered strip.
In the alternative gas atomisation process, the cooling rate of the molten droplets produced during atomisation is sufficiently slow for the surface tension forces to spheroidise the particles before solidification. Where materials having relatively slow freezing points are required, e.g. braze materials, this effect is exaggerated.
Gas-atomised powders are generally more widely available than water atomised powders and also tend to contain less impurity since they are conventionally atomised using pure inert gases such as argon. Water atomised powders are more likely to bo oxidised or otherwise contaminated by dissociation products of water, or any dissolved impurities the water may contain.
There are, therefore, advantages which would accrue from the use of gas-atomised powders for the production of certain strip products where the absence of impurities is important, e.g. strips for use in brazing applications if problems associated with compaction and sintering of strip produced from gas-atomised powders can be overcome. One particular problem which does not occur during the roll compaction process arises as a consequence of the fact that spherical powder particles produce a strip in which the particle content tends to "flow" producing large extensions with relatively little particle interaction. Hence the green strength of the compacted strip and surface area contact of the particulate content of the strip are both low resulting in a strip having inadequate physical properties following first compaction and first sintering.
The present invention sets out to provide a process in which flat products can be produced from a slurry containing spherical gas atomised powders.